The present disclosure relates to non-volatile memories using variable resistance elements and read circuits of the memories, and more particularly to techniques providing low voltage operation at low power consumption.
Conventionally, NAND flash memories and NOR flash memories using floating-gate transistors or MONOS transistors are largely used as non-volatile memories. In recent years, as next-generation non-volatile memories following these memories, resistance variable non-volatile memories such as spin transfer torque magnetoresistive random access memories (STT_MRAMs), resistance RAMs (ReRAMs), and phase change RAMs (PRAMs) have drawn attention.
Data in resistance variable memories is rewritten by flowing rewrite currents to variable resistance elements to change the electrical resistance states of the variable resistance elements. A high resistance state is called HRS, while a low resistance state is called LRS. Variable resistance elements, to which currents flow in a single direction to change the resistance states, are called a monopolar type. Variable resistance elements, to which currents flow in both directions, i.e., which change the directions of current flow to change the resistance states, are called a bipolar type.
Data in resistance variable memories is read by flowing read currents to variable resistance elements to detect the resistance of the variable resistance elements. The number of read currents is set smaller than that of rewrite currents. As compared to flash memories, resistance variable memories perform rewriting at high speed and read operation at a low voltage. Therefore, in recent years, reading at a low voltage of 1 V or less with low power consumption utilizing this feature has been increasingly expected.
Read circuits determine digital values of data by comparing data written in variable resistance elements, i.e., the resistance states of the variable resistance elements, to the state of reference resistance, which may also be a current or a voltage. However, a serious problem of resistance variable memories is that the resistance states (i.e., the resistance values) vary depending on the number of rewriting, or that the resistance states vary temporally.
Read circuits need to accurately determine the digital values of memory cells, even when the resistance values of variable resistance elements vary within a wide range. That is, for example, even if sufficient margins are obtained between the resistance states of the variable resistance elements and a reference resistance, for example, immediately after rewriting data to the memory cells, or even when margins are hardly obtained, for example, when the read circuits are close to the end of lifetime. When the resistance of a variable resistance element is close to reference resistance, whether or not read circuits accurately and stably determine the resistance of the variable resistance element is important as a parameter for determining data holding characteristics of a non-volatile memory.
In response to the demand for the reduction in voltages, read circuits have the following problems. Read circuits convert resistance differences between memory cells and reference cells to currents or voltages, and amplify the currents or voltages with amplifiers to determine digital values. In read circuits, the resistance differences decrease with decreasing supplied voltages. Then, conversed values from the resistance differences to voltages etc., become small, and voltages supplied to transistors in the amplifiers become low. This reduces the gain of the amplifiers, and increases mismatch between pairs of transistors, thereby reducing the speed and the accuracy in reading. If the voltages further decrease, malfunctions of the read circuits increase, and at worst, the functions of the read circuits may stop.
A load circuit, which is a conventional current mirror circuit, and a differential amplifier using a differential transistor pair (i.e., a transconductance amplifier) are considered as a circuit which controls the lower limit operating voltage of a read circuit. In this circuit, the voltage needed for holding the current mirror circuit in a saturation state is obtained by the expression Vth+Vds_sat, where the threshold voltage of the transistors is Vth, and the drain voltage for holding the transistors in the saturation states is Vds_sat. In addition, since the voltage for holding the differential amplifier in the saturation state is obtained by the expression 2×Vds_sat, the minimum operating voltage Vdd_min of this circuit is expressed by the following equation.Vdd_min=Vth+3×Vds_sat
For example, in a process with Vth of 600 mV and Vds_sat of 200 mV, the minimum operating voltage Vdd_min is 1.2V. In order to set the minimum operating voltage Vdd_min to 1 V or lower, a process with much lower Vth and Vds_sat is needed.